Add support for Win64 stack unwind data as STACK CFI

Add support for Win64 stack unwind data as STACK CFI

This is a copy of https://breakpad.appspot.com/1264002/ where code review took
place.

See https://bugzilla.mozilla.org/show_bug.cgi?id=548035 and
https://code.google.com/p/chromium/issues/detail?id=115922

Credit to Mikhail I. Izmestev <izmmishao5@gmail.com> for original patch in
https://breakpad.appspot.com/345002/

BUG=572
TEST=Run dump_syms_unittest after compiling dump_syms_regtest.cc with x64
toolchain
R=mark@chromium.org

Committed: https://code.google.com/p/google-breakpad/source/detail?r=1290

[Will Harris]

This is a copy of https://breakpad.appspot.com/1264002/ with the correct base
for gcl to work correctly.

[Mark Mentovai]

LGTM

[Will Harris]

Committed patchset #2 manually as r1290 (presubmit successful).